Cause of the Crusades - 3000 Christian Pilgrims massacred in JerusalemJerusalem had fallen into the hands of the Seljuk Turks. Christian pilgrims were no longer welcomed in Jerusalem. 3000 pilgrims were massacred in Jerusalem by the Turks and Christians across Europe had now realized that Jerusalem had fallen into new hands. The Turks destroyed the Christian churches of Jerusalem or made them into stables for their horses.
Cause of the Crusades - The Preaching of Peter the Hermit
After being commisioned by Pope Urban II, Peter the Hermit preached the crusade. He traversed Italy and France and spoke anywhere where people would listen. He touched people with sympathy and fired them up with teachings of the suffering of their brethren at the hands of the infidels.

Cause of the Crusades - The Threat of the Turks
While Peter the Hermit had been preaching in the West, the Turks had been advancing eastward to Constantinople. Because of this, the Greek emperor Alexius Comnenus asked Pope Urban II for help to fight them off. If he would not do this, then the city and its Holy relics would fall to the Turks.

Cause of the Crusades - Pope Urban II & the Council of Clermonthttp://www.middle-ages.org.uk/cause-of-crusades.htm